This present invention relates generally to electronic article security systems for detecting the presence of a security tag within a detection zone and more particularly to an improved pulse-listen electronic article security system employing pseudo-random frequency/time hopping RF bursts to provide a reduced false alarm rate.
The use of electronic article security (EAS) systems for detecting and preventing theft or unauthorized removal of articles or goods from retail establishments and/or other facilities such as libraries has become widespread. In general, such EAS systems employ a security tag, which is detectable by the EAS system and which is secured to the article to be protected. Such EAS systems are generally located at or around points of exit from such facilities to detect the security tag, and thus the article, as it transits through the exit point.
Due to environmental and regulatory considerations, individual EAS systems are generally effective over only a limited area in which a security tag attached to a protected article may be reliably detected. Such area, typically referred to as a detection zone, is generally limited to about six feet in width. While many stores and libraries have only a single exit doorway of a size commensurate with such a six foot wide detection zone, many other retail establishments have eight or ten exit doorways arranged side by side and may also have a multiplicity of separate exits. Furthermore, large mall stores frequently have a generally wide open area or aisle of ten feet or more in width serving as a connection with the mall. Thus, in many such situations, a plurality of EAS systems are required to fully protect either a multiplicity of separate exit points and/or individual exit/entrance points having an exit width greater than that which can be reliably protected by a single EAS system.
One type of EAS system which has gained widespread popularity utilizes a security tag which includes a self-contained passive resonant circuit in the form of a generally planar printed circuit which resonates at a predetermined frequency. Typically, an EAS system for detecting such a resonant circuit security tag includes a transmitter which transmits electromagnetic energy into the detection zone to form an electromagnetic field having frequency components proximate to the resonant frequency of the security tag. Such an EAS system also includes a receiver to detect the electromagnetic field within the detection zone. When an article having an attached security tag moves into or passes through the detection zone, the security tag is exposed to the transmitted electromagnetic energy, resulting in the security tag resonating to provide an output signal, thereby disturbing the electromagnetic field within the detection zone. Such disturbance is detectable by the receiver. The detection of such field disturbance by the receiver indicates the presence of an article with a security tag within the detection zone and the receiver activates an alarm to alert security or other personnel.
Because of the manufacturing techniques to produce them, the resonant frequency of a typical resonant security tag may vary by plus or minus ten percent or more from the nominal design resonant frequency of the tag. In order to reliably detect the presence of a security tag in the detection zone, EAS systems generally transmit a range of frequencies in order to ensure that a frequency component from the transmitted signal falls proximate to the resonant frequency of the security tag.
A popular type of EAS system, generally called a pulse-listen type EAS system, manufactured by Checkpoint Systems, Inc. of Thorofare, N.J. and known as the Strata.TM. System, repeatedly transmits a sequence of RF burst signals of electromagnetic energy at different frequencies such that the frequency of at least one of the bursts falls near the resonant frequency of a security tag to be detected. The EAS system gates the transmitter off between the bursts and enables the receiver during quiescent periods of time between the transmitter bursts. The receiver detects a security tag located within the detection zone by detecting the energy re-radiated by the resonant security tag during the quiescent periods.
Prior art pulse-listen EAS systems such as the Strata.TM. System provide for highly reliable detection of security tags within a detection zone by requiring that the receiver register a prescribed number of tag detections over a predetermined number of transmitted burst signal repetitions. However, where co-located EAS systems employ a common burst frequency/time pattern there is a potential for one EAS system to detect transmitted bursts from another EAS system, giving rise to undesired false alarms or reduced detection sensitivity. A satisfactory method for eliminating false alarms from co-located EAS systems, is to synchronize the transmitters of all co-located EAS systems to ensure that no transmitted burst overlaps the receive quiescent period of any receiver. A typical method of synchronization employs connecting cables between a single master EAS system and all other EAS systems which serve as slave systems. However, connecting cabling is costly and sometimes impractical to install. Alternatively, as described in U.S. Pat. No. 4,667,185, synchronization may be performed by wireless methods. However, wireless systems require additional complex synchronization circuitry. Additionally, synchronization is largely ineffective against interference from co-located EAS systems of other manufacturers and from other external interference.
The present invention eliminates the need for synchronization between co-located EAS systems by having each co-located EAS system utilize a distinct pseudo-random frequency/time pattern for interrogating security tags within an associated detection zone. By selecting the distinct frequency/time patterns such that the frequency/time patterns appear to be randomly distributed and have a cross correlation between themselves that is small, the probability of transmitter bursts from any EAS system causing a false alarm in any other co-located EAS system is extremely small. Further, because of the pseudo-random frequency/time pattern of reception the present invention provides a high degree of interference rejection to interfering signals generally.